Personal tracking devices have been found to be useful in locating lost objects and, more importantly, missing persons. Such tracking devices typically use a network of Global Positioning Satellites (GPS) in low earth orbit that broadcast precise timing signals from on-board atomic clocks. Using triangulation formulas, a device that picks up signals from several satellites simultaneously can determine its position in global coordinates, namely latitude and longitude. Thus, an object and/or person carrying the GPS device may be located provided the appropriate equipment and trained personnel are available for determining the location of the GPS device. However, GPS signals, like any other satellite signal, are prone to numerous interferences including atmospheric disturbances, such as solar flares and naturally occurring geomagnetic storms. In addition, man-made interference can also disrupt, or jam, GPS signals. Further, anything that can block sunlight can block GPS signals. This raises the question of whether or not GPS is reliable in locating a missing and wandering person who may be in, or next to, a building, under a tree, in the brush, under a bridge, in an urban environment, in a vehicle or even a person who has fallen down and has their GPS unit covered by their own body.
Other known tracking devices use radio signal emitting transmitters. However, these types of tracking devices require an expensive receiver device in the area to receive and track the emitted radio signal. Thus, without the appropriate receiving device in the area and/or trained personnel capable of operating the receivers, these tracking devices would be useless for locating lost objects and/or missing persons.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and OFDM wireless systems, as well as code-division radio communications systems such as CDMA (IS-95, IS-2000) and Universal Mobile Telecommunications System (UTMS), the latter of which is also known as W-CDMA. The Global System for Mobile Communications (GSM) model discussed above is an exemplary but not exclusive environment in which the present invention may be used.